1. Field of the Invention
The present invention relates to a BGA (Ball Grid Array) type semiconductor package and semiconductor device.
2. Description of the Prior Art
Semiconductor chips such as LSIs are used in various electric/electronic apparatuses, e.g., computer apparatuses such as personal computers, portable electronic apparatuses including portable telephones, office home electric products such as printers, television sets, video cassette recorders. In use of a semiconductor chip, the semiconductor chip is packaged and then mounted on a printed board to constitute a semiconductor device, thereby being incorporated in the various apparatuses. The electric connection between the semiconductor device and the apparatus incorporated therewith is performed with pins which are conventionally arranged on the semiconductor device as input/output terminals.
However, with an increase in integration density of a recent LSI, a package on which the LSI is mounted must be decreased in size with an increase in number of pins and a decrease in pin pitch. Accordingly, the type of packaging has shifted from the above conventional pin insertion type to a surface mounting type such as a QFP (Quad Flat Package) type or an SM-PGA (Surface Mount type-Pin grid Array).
However, in these surface mounting type packages using pins and leads, the lower limit of a terminal pitch is high. For example, a pitch lower than 1.27 mm cannot be easily obtained in the PGA type package, and a pitch lower than 0.3 mm cannot be easily obtained in the QFP type package. Further, when a high-speed signal is to be processed, a large inductance component is generated at pin and lead portions. For this reason, a signal is disadvantageously reflected at a high frequency, and an increase in delay disadvantageously occurs.
In order to solve the above problem, a BGA (Ball Grid Array) type package is proposed. The BGA type package has the following characteristic feature. That is, a bump is formed with a solder ball or the like at an input/output portion of a package substrate, and the bump is melted to connect the input/output portion of the package to a terminal of a printed board. According to this package, a pitch which is narrower than that of a package using pins or leads can be achieved. Therefore, an increase in number of pins and a decrease in size can be achieved.
A conventional BGA type package, as shown in FIG. 1, has a cavity-down structure in which a cavity used for mounting a semiconductor chip 1 on a package substrate 3 faces a printed board 5, and the electric connection between the package substrate 3 and the printed board 5 is arranged on the same side as that of the cavity of the package substrate 3. A wiring pattern 7 is formed around the cavity of the package substrate 3. The semiconductor chip 1 and the wiring pattern 7 are connected to each other with bonding wires 9, and the wiring pattern 7 and the printed board 5 are connected to each other with solder balls 11.
In recent years, increasing of a frequency as one manner of obtaining a high-speed and high-performance electronic apparatus tends to further advance, and the operation frequency of a semiconductor element also increases. Since such a high-frequency semiconductor element has a high output, a countermeasure against an increase in heat generation with the high output is a problem. As a result, improvement of heat radiation properties must be achieved even in a BGA type package.
In the BGA type package as shown in FIG. 1, when heat generated by the semiconductor chip is radiated toward the printed board, the heat is transmitted through the package substrate and the solder balls to the printed board. However, such a long heat-transfer path has a high thermal resistance. Therefore, the BGA package cannot sufficiently cope with an increase in heat generation of the semiconductor element described above, and the heat radiation properties must be therefore improved.
In addition, when the semiconductor package is considered as a whole, the resistance of the wires arranged on the package substrate also serves as a heat generator, and the resistance of the wires must be decreased to reduce the heat generation. In order to decrease the resistance of wires, it is necessary to make the wires short, to make large the sectional area (i.e., the diameter or the thickness) of each wire, or to form the wires with a low-resistance material. However, in order to use shorter wires, a technique for forming finer wires must be developed, and this causes a disadvantageous increase of manufacturing costs. Use of a thick wire is improper for a high-density wiring structure, and makes it difficult to print a fine wiring pattern. In many cases, the thick wire is a disadvantage.